The overall objective is to continue the development of an imaging modality ("Electrocardiographic Imaging") for the reconstruction of cardiac electrical, activity from potentials measured away from the heart (i.e. on the torso surface and in the blood cavity), and to further our understanding of the relationship between cardiac excitation and the electric potentials that it generates. Standard electrocardiographic techniques (i.e. ECG and VCG) can not provide information on regional cardiac electrical activity and on the location of electrical events (e.g. an arrhythmogenic focus) in the heart. Potential distributions and isochrone maps on the epicardial and endocardial surfaces of the heart mirror regional and local electrophysiological events in the myocardium and can be computed from the potential distributions measured over the torso surface and over the surface of an intracavitary catheter - probe. Goals for the next grant period are: (l) To continue the development of mathematical methods for the reconstruction of potentials and of activation isochrones on the epicardial and endocardial surfaces of the heart from body surface potentials and intracavitary potentials, respectively; (2) To test the ability of the epicardial - and endocardial - reconstruction procedures to detect and locate single and multiple foci of arrhythmogenic activity; (3) To characterize the spatial resolution of these reconstruction procedures; (4) To test the ability of these procedures to determine the location and extent of an infarcted region; (5) To begin implementation in the clinical setting and to test the approach in selected groups of patients that provide an opportunity to validate the reconstruction procedure (WPW patients, patients with epicardial pacing wires, patients with implanted pacemakers, patients with right or left bundle branch block, and patients who undergo epicardial mapping during surgery). (2), (3) and (4) above will be conducted in an isolated canine heart and a human-shaped torso tank setup. Health relatedness: The availability of potential distributions and activation sequences over the surfaces of the heart will provide diagnostic information regarding electrical disturbances (e.g. conduction abnormalities, types of arrhythmias, location of reentry pathways and of the areas of slow conduction within these pathways). It will also help evaluate effects of interventions (e.g. antiarrhythmic drug therapy, neural changes) on cardiac activity and arrhythmogenesis. In addition, it will provide non-surgical means for locating foci of arrhythmogenic activity prior to surgical or catheter ablation.